1. Field of the Invention
The present invention generally relates to modular inductance coils and, in particular, to air core coils of the type used for impedance matching of high power radio transmission antennas such as for use in the normal high frequency range, such as 1.5 to 30 MHz.
2. Description of Prior Art
High power radio transmitters and receivers (hereinafter transceivers) generally include a low power transceiver connected to some form of input, a high power amplifier, an antenna tuner and an antenna. In such prior art systems, the tuner has the important function of matching the antenna impedance to that of the high power amplifier for enabling the most efficient power transfer therebetween. Difficulty in accomplishing this function may result from the wide frequency band which causes the antenna impedance to vary greatly, according to frequency. For example, the impedance of a 5 meter long whip antenna (as represented in complex number form) can vary between 3-j1500 at 2 MHz and 800+j900 at 20 MHZ.
According to a prior art conventional technique which is widely used to compensate for variations in antenna impedance, an impedance matching network is employed which is composed of continuously variable coils and capacitors operated by servo-motors. Such a network is complex and the requisite tuning is accomplished by sophisticated closed-loop servo systems which are quite anachronistic in modern electronic equipment and which suffer from serious limitations such as (1) low reliability owing to the use of moving parts; (2) excessive tuning time (rarely less than 10 seconds, and often in high power equipment exceeding one minute); and (3) difficult maintenance due to system complexity and the sophistication of the individual parts thereof.
In a prior art effort to overcome the disadvantages of this servo motor approach, the motors have been replaced with high frequency relays thus producing static networks. According to this prior art approach, variable inductance and capacitance is provided by a finite set of reactive elements connected through these relays. These reactances can assume a discrete number of impedance values thus allowing the closed-loop control of the network to become discrete instead of continuous as in the previous prior art approach. As a result, any consequent inaccuracy in the requisite impedance matching can be reduced to acceptable limits.
Included in this latter approach is a step variable inductance usually consisting of "n" inductances with impedance values in binary progression and connected in series. Each coil is shunted by a relay contact in order to control the coil connection. This form of variable inductance can provide 2.sup.n values of impedance at a given frequency, where "n" equals the number of inductances.
Unfortunately, a plurality of coils has a much higher volume than a single coil for a given inductance value and technology level. This is not considered a problem for low power matching networks such as those having a power of less than 100 watts, because in such a case small ferrite-core coils can be used. However, when air core coils are employed, such as those which must be used for higher power systems and which are configured as toroidal coils and single layer solenoids, size problems are presented which are not easily overcome through merely using a plurality of inductance elements. In an effort to overcome these problems, the prior art has attempted to reduce the volume required for the air core coils by employing a multilayer solenoid configuration. While this solution is theoretically very efficient, in practice it is very difficult to execute. In this regard, by way of example, a three layer solenoid could be obtained by winding one layer on each of three coaxial supports assembled in a complex support structure. Each winding, in the shape of a solenoid, would be made by a conductor with circular section and contained in suitable grooves. The three windings would be connected in series in order to generate magnetic flux in the same direction. In order to provide an idea of the possible size reduction in such arrangement, it should be noted that the inductance of a three layer solenoid could be considered, with very rough approximation, to be nine times higher than the one constituted by a single central layer. However, in spite of this advantage, the preparation of such a multilayer solenoid is quite difficult because of serious difficulty in constructing the required complex support, difficult in assembly of the windings, a lack of flexibility due to only a few values of inductance being feasible for a particular optimum size support, difficult cooling of the assembly, and difficult coil assembly if a step variable inductance is required. These disadvantages of the prior art are even more apparent when such a coil assembly is to be employed for an antenna tuner since multilayer solenoids having as many as five to ten layers, as opposed to the three referred to above, would normally be required.
These disadvantages of the prior art are overcome by the present invention.